Manufacturing processes often include steps in which one or more relatively small objects must be correctly positioned on a relatively larger object, e.g., to thermally mold a coating material onto the smaller elements. Many conventional molding systems employ two cooperating die molds which are formed to have respective recesses that define around the small elements corresponding spaces into which molten encapsulating material is caused to flow under pressure for the desired encapsulation. The encapsulating material typically is a resinous material which flows at a heated temperature and becomes solid when cooled.
Incorrect positioning of the small elements which are to be encapsulated can easily obstruct adequate clamping of the die molds to each other and may consequently allow the molten material, in its pressurized and molten state, to flow in an uncontrolled manner across and between the cooperating die molds. A known term for this kind of problem, where a resinous material is to be molded, is "resin spread". The problem is particularly acute where a number of small elements, e.g., lead frames onto which electronic components have been formed, are to be placed on a lower die mold at precise locations.
In high-speed automated molding systems, errors can occur not only when individual lead frames are incorrectly positioned but sometimes also due to inadvertent disposition of one lead frame above another lead frame. Whatever the reason, when "resin spread" occurs as a consequence, at least four wire-bonded lead frames are typically lost from production. Even worse, the molding system may have to be shut down, often for four hours or longer, to permit sufficient cooling thereof to allow workers to clean the die surfaces exposed to the resin spread. In more extreme circumstances, e.g., where the mislocation of a lead frame is severe or one lead frame is placed over or in an overlapping relationship relative to another lead frame, there may even be actual physical damage to one or both of the cooperating die molds.
It is generally difficult to detect the presence of a lead frame on a typical die surface for various reasons. Both are made of metal, and hence a device that utilizes magnetism, e.g., an eddy sensor, cannot work satisfactorily to distinguish between a lead frame and the mold die. Furthermore, both the lead frame and the die mold surface on which the lead frame typically rests have similar reflectivities, hence devices such as cameras which utilize light in the visible spectrum cannot operate to distinguish between a lead frame and a die mold.
Other factors which must be considered include the fact that any sensors which are to be employed must be mounted on the inloader head of the apparatus and must be capable of operating while being subjected to relatively stressful repetitive movement. In addition, the environment is one in which the temperature typically is 180.degree. C. above and below the inloader head, which is hazardous for human beings and inimical to many electronics components. Also, rapid detection of correct placement of the lead frame on the die mold is required so that the cycle time, i.e., time between successive molding operations, is not unduly prolonged.
These requirements of reliability, durability, tolerance of high temperatures, and speed in making the necessary determinations, all pose significant challenges. The present invention provides a satisfactory solution at very moderate cost and without the need for complex, fragile, or unduly sensitive components.
Various types of known manufacturing processes also require correct placement, location, or formation of elements for successful production of final products which physically include the elements.
U.S. Pat. No. 5,186,541, to Paulk, titled "Non-Contact Infrared Temperature Sensing System", teaches a non-contact infrared temperature sensing system which employs an infrared detector and compares the emissivity of a target to an inputted value to determine the temperature of the target.
U.S. Pat. No. 5,165,791, to Miki et al, titled "Method and Apparatus for Measuring Temperature Based on Infrared Light", discloses a method and apparatus for utilizing infrared light to measure the temperature of a semiconductor formed with a surface layer consisting of two kinds of materials which have different emissivities and optical reflectances. The disclosed method involves the taking of an image with diffused light reflected from the surface of the semiconductor element and of determining an area ratio in which each of the two kinds of materials occupy the surface of the semiconductor element by comparing relative average brightness values.
U.S. Pat. No. 4,973,948, to Roberts, titled "Reversed or Missing Lead Frame Detector", teaches the use of a sensing means to determine the presence or absence of a unique aperture by completing or closing an electrical circuit to generate an indication or an alarm if a problem exists. The disclosed system employs a steel reflector pin engaging a corresponding aperture in the lead frame to open or close a circuit. When the circuit resistance is not within an expected range of resistance values, dependent on the total number of lead frames being tested, the user will receive a corresponding signal.
U.S. Pat. No. 4,481,418, to Vanzetti et al, titled "Fiber Optic Scanning System for Laser/Thermal Inspection", discloses the use of a plurality of different infrared radiating elements including an optical head movable in two dimensions above a fixed device to sequentially scan the plurality of infrared radiating elements. The system employs a laser beam source connected to a movable head for projecting thermal energy onto the infrared radiating elements.
None of these references is considered capable of providing either an apparatus or a method for quickly and accurately determining whether there is correct placement of one or more lead frames on a die mold which initially may be at an elevated temperature, e.g., 180.degree. C. There is clearly a need for a sturdy, simple and reliable apparatus and a corresponding method for solving this problem.
The present invention, as described more fully hereinbelow with reference to the accompanying figures, is intended to provide an effective solution.